Various ink jet printing technologies have been or are being developed. One such technology, referred to hereinafter as acoustic ink printing (ALP), uses acoustic energy to produce an image on a recording medium. While more detailed descriptions of the AIP process can be found in U.S. Pat. Nos. 4,308,547, 4,697,195, and 5,028,937, essentially, bursts of acoustic energy focused near the free surface of a liquid ink cause ink droplets to be ejected onto a recording medium.
As may be appreciated, acoustic ink printers are sensitive to the spatial relationship between the acoustic energy's focal area and the ink's free surface. Indeed, current practice dictates that the focal area be within about one wavelength (typically about 10 micrometers) of the free surface. If the spatial separation increases beyond the permitted limit, ink droplet ejection may occur poorly, intermittently, or not at all.
While maintaining the required spatial relationship is difficult, the difficulty increases as droplet ejection rates change. This is because experience has shown that high droplet ejection rates cause a spatial change in the static level of the ink's free surface. This is believed to be a result of the rather slow rate of decay of mounds raised on the free surface from which droplets are ejected. Thus, in the prior art, the spatial relationship between the acoustic focal area and the ink's free surface is, undesirably, a function of the droplet ejection rates. This dependency is a problem in high speed AIP since droplet ejection rates vary as an image is produced. While the spatial variation depends upon such factors as the liquid's viscosity, the acoustic energy used to eject a droplet, and the density of droplet ejectors, static height variations about equal to the acoustic wavelength are encountered in practice. Therefore, techniques that stabilizes the spatial relationship between the acoustic focal area and the ink's free surface would be beneficial.